In the fabrication of integrated circuits (ICs), it is often necessary to polish a side of a part such as a thin flat wafer of a semiconductor material In general, a semiconductor wafer can be polished to remove topography, surface defects such as crystal lattice damage, scratches, roughness, or embedded particles of dirt or dust. This polishing process is often referred to as mechanical planarization and is utilized to improve the quality and reliability of semiconductor devices. This process is usually performed during the formation of various devices and integrated circuits on the wafer.
In general, the mechanical planarization process involves holding or rotating a thin flat wafer of semiconductor material against a wetted polishing surface under a controlled pressure or temperature. A polishing slurry such as a solution of alumina or silica is utilized as the abrasive medium. A rotating polishing head is typically utilized to hold the wafer under controlled pressure against a rotating polishing platen. The polishing platen is typically covered with a relatively soft wetted material such as blown polyurethane.
Such apparatus for polishing thin flat semiconductor wafers are well known in the art. U.S. Pat. Nos. 4,193,226 and 4,811,522 to Gill, Jr. and U.S. Pat. No. 3,841,031 to Walsh, for instance, disclose such apparatus.
A particular problem encountered in the use of such polishing apparatus is in the determination that a part has been planed to a desired flatness or relative thickness. In the past, this typically has been accomplished by control of the rotational speed, downward pressure, and polishing time of the planarization process. As a final step, however, the part typically must be mechanically removed from the polishing apparatus and physically measured by techniques known in the art to ascertain dimensional and planar characteristics of the polished part. If the part does not meet specification, it must be loaded back into the polishing apparatus and planarized a second time. Alternately, the part may have been subjected to too much polishing and an excess of material may have been removed, rendering the part as substandard.
Additionally, the semiconductor wafer may be subjected to spatially non-uniform planarization due to the relative velocity differential between the outer peripheral portions and the interior portions of the rotating semiconductor wafer. The faster moving peripheral portions of the semiconductor wafer may, for instance, experience a relatively larger rate of material removal than the relatively slower moving interior portions. In the past, this problem has been approached by the use of a polishing head having a generally convex shape to impart a greater force on the interior portions of the semiconductor disc and a lesser force along the outer peripheral portions.
These planarization problems are compounded because the semiconductor wafer is held face down against the polishing platen; and, absent removing the semiconductor disc, there is no provision for monitoring the polishing process.
In general, there is a need in the mechanical planarization of semiconductor wafers to be able to detect or monitor the endpoint of the planarized wafer while the planarization process is in operation. The present invention is directed to a novel method and apparatus for endpoint detection of a semiconductor wafer which can be accomplished during the planarization process.